An expression of milk from a female breast during breast feeding is preceded by a milk ejection reflex (MER), in which a contraction of the myoepithelial cells around milk glands in the breast forces milk to flow towards the nipple. Until an MER occurs, little or no milk will be expressed.
An MER is naturally instigated by the suckling action of a baby. After an MER has occurred, a baby will naturally alter its suckling action to one which encourages a continued flow of milk from the breast.
It is desirable that an artificial breast pump should not only be able to instigate an MER but should also be able to mimic the second stage of the natural suckling process by altering the type of stimulation provided to the breast after an MER has occurred. However, in order to implement the change in stimulation at the correct time, it is first necessary to detect when an MER has occurred.